NASA/university
study finds aggressive conservation helped region's aquifer rebound quickly from
one of the worst droughts in California history

Underground
water reserves in California's Silicon Valley rebounded quickly from the
state's recent severe drought, demonstrating the success of aggressive
conservation measures, according to a new space-based study by NASA and
university scientists.

Using satellite
data from COSMO-SkyMed, a constellation of four Italian Space Agency (Agenzia
Spaziale Italiana, or ASI) satellites, a research team led by Estelle Chaussard
at the University at Buffalo in New York, and including scientists from NASA's
Jet Propulsion Laboratory in Pasadena, California, used a technique called
synthetic aperture radar interferometry to monitor the entire Santa Clara
Valley aquifer near San Jose from 2011 to 2017. This type of radar can capture
the subtle up-and-down movements of Earth's surface of just minute fractions of
an inch (a few millimeters) that occur when water levels rise or fall
underground. The scientists used hundreds of radar images obtained under a
license from ASI to calculate how much the land surface elevation changed over time.
The measurements show the aquifer began to rebound in late 2014, when the
drought was still going strong, and that groundwater levels had returned to
pre-drought levels by 2017, thanks to conservation measures that intensified in
2014, and heavy winter rains in 2016.

During the
2012-15 drought, the Santa Clara Valley Water District employed an array of conservation measures. These included restricting sprinkler
use and asking customers to take shorter showers and convert lawns and pools
into less-thirsty landscapes. The district also imported water from outside the
region.

Chaussard says
the actions may have helped stave off irreversible damage to the aquifer, which
measures about 212 square miles (550 square kilometers) and lies beneath a highly
urbanized area. She explains when groundwater levels reach a record low, the
porous sands and clays in which the reserves reside can dry up so much that the
clays don't retain water anymore. The new study shows that thanks to the
intensive water management efforts, this did not happen in the Santa Clara
Valley.

Chaussard says
the aquifer monitoring method her team used can work anywhere where there are soft-rock
aquifer systems and where synthetic aperture radar satellite data are
available, including in developing nations with few resources for monitoring.

"We wanted to
see if we could use a remote sensing method that doesn't require ground
monitoring to understand how our aquifers are responding to a changing climate
and human activity," she says. "Our study further demonstrates the utility of synthetic
aperture radar interferometry, which scientists also use to measure surface
deformation related to volcanoes and earthquakes, for tracking ground
deformation associated with changes in groundwater levels."

"This study further
demonstrates a complementary method, in addition to
traditional ground-based measurements, for water management districts to
monitor ground deformation," added JPL co-author Pietro Milillo. "The technique marks an improvement over
traditional methods because it allows scientists to gauge changes in ground
deformation across a large region with unprecedented frequency." He said the
COSMO-SkyMed satellites provided information for the aquifer as often as once a
day.

Underground
stockpiles of water -- housed in layers of porous rock called aquifers --
are one of the world's most important sources of drinking water. Some 2.5
billion people across the globe rely on aquifers for water, and many of these
repositories are being drained more quickly than they can be refilled,
according to the United Nations Educational, Scientific and Cultural
Organization.

"To monitor
aquifers, you need a lot of measurements in both space and time," she says. "Sampling
water levels at wells may give you a continuous time series, but only if they
are constantly monitored, and automated monitoring may not be common. Also,
even a high density of wells may not adequately capture basin-wide spatial
patterns of water storage, which is key to understanding processes at stake."

The methods employed
in this study provide a more complete picture of how an aquifer responds during
a drought and how water conservation methods can have a real and positive
impact on sustaining the health and viability of pumped groundwater aquifers.
The satellite radar imagery not only fills in data gaps between wells, but provides
valuable insights into how aquifers are responding beyond the edges of
monitoring well networks so that water agencies can more effectively manage
their precious resources.

The upcoming
NASA-ISRO (Indian Space Research Organisation) Synthetic Aperture Radar (NISAR)
satellite mission, planned for launch in 2021, will systematically collect
radar imagery over nearly every aquifer in the world, improving our
understanding of valuable groundwater resources and our ability to better
manage them. In addition to tracking groundwater use in urban settings, NISAR
will be able to measure surface motion associated with groundwater pumping and
natural recharge in rural communities, in areas with extensive agriculture, and
in regions with extensive vegetation, conditions that are typically more
challenging.

The research was
published Sept. 25 in the Journal of Geophysical Research - Solid Earth. Other participating
institutions include the University of California, Berkeley; Purdue University,
West Lafayette, Indiana; and the Santa Clara Valley Water District.